I've built several but not as short as 23". You'll have to play with focal lengths to make it turn out right. Don't know what Unertl used but I have always used 48 TPI.
                                                         Rich

if you look in general discusion page 10 on scope design.  this should help you.

I read somewhere of 40 tpi, but it was not on an unertl. I know my winchester 8X and my Lyman 438 are not the same. 

If you use a two lens achromatic erector set in the middle, you cut the focal length in half. Example; if you use two 120mm fl lenses facing each other, you get 60mm focal length, or there abouts when considering lens thickness, etc. This may help "shorten" your scope. ( flat parts facing out and curved parts facing each other)

If you move the erectors a slight bit closer to the objective you can slightly boost magnification.

Some of this is experience, some is read knowledge. So, buyer beware.

I have a large, unmarked scope end or booster I am trying to make into a complete scope. I have tried the three lens theory and decided to try the five lens. It seems to give more flexibility to "adjust" the overall length amongst other things.

I have done many web searches about building terestrial telescopes and found an abundance of info that way. Well worth the effort.

Merwin